The present invention relates to a process for the preparation of superconducting compound materials formed of substrates and metallic layers of metastable or unstable phases strongly adhering to the surface of the substrates, the layers each containing more than one element and being deposited onto the substrate surfaces from a gas- or vapor-phase.
In known processes for the preparation of metallic layers of metastable or unstable stoichiometric compounds, especially phases with an A15 crystal structure, the elemental components of the compounds are condensed onto substrates which have already been heated to considerably high temperatures necessary for the formation of the metastable compounds. For example, in the preparation of the metastable A15 compound Nb.sub.3 Ge, niobium and germanium have been deposited on heated substrates, at a temperature of about 1073.degree. K. In this way, with certain process conditions being observed, metastable phases with superconducting properties can be produced which exhibit a reproducible transition or critical temperature T.sub.c of about 21.degree. K.
There have been many attempts to obtain very high transition temperatures, around 23.degree. K. Indeed, on occasion, superconducting layers with transition temperatures between 22.degree. and 23.degree. K. have been obtained, but the preparation of these layers was not reproducible. It is suspected that the A15 phase responsible for the superconductivity would be stabilized by addition of impurities during the process of growth, but this assumption was not sufficient to understand the processes involved in the formation of, for example, a Nb.sub.3 Ge-phase, especially at the interface with the substrate.
It has been found that Nb.sub.3 Ge-films with a thickness of 100 nm or less, condensed onto a substrate surface by means of cathode sputtering, no longer show the maximum attainable transition temperature between 22.degree. K. and 23.degree. K., which has been measured in thicker films. See J. R. Gavaler et al, Applied Physics Letters, 33, No. 4, 1978, pp. 359 to 361. While in Nb.sub.3 Ge-films of a thickness less than 100 nm on sapphire substrates, transition temperatures (T.sub.c) between 14.degree. and 17.degree. K. and unexpectedly large lattice parameters have been established, only smaller lattice parameters and transition temperatures of about 22.degree. to 23.degree. K. could be determined with increasing growth of the film thickness. After data from different laboratories became available, showing that the presence of impurities during the growth process of the A15 phase had a beneficial effect on the formation of stoichiometric high-T.sub.c Nb.sub.3 Ge, Gavaler et al investigated various substrates laminated with niobium-germanium. These substrates have included sapphire, Al.sub.2 O.sub.3, Hastelloy B, foils of tantalum and vanadium and, in some cases, foils of tantalum and vanadium oxidized on their surface with air at 873.degree. K. The following transition temperatures were found in these experiments:
______________________________________ Nb.sub.3 Ge on pure Ta-substrate T.sub.c about 13.degree. K. Nb.sub.3 Ge on oxidized Ta-substrate T.sub.c about 21.5.degree. K. Nb.sub.3 Ge on vanadium-substrate T.sub.c about 13.degree. K. Nb.sub.3 Ge on Al.sub.2 O.sub.3 -substrate T.sub.c about 22.degree. K. ______________________________________
The yet unresolved difficulties with respect to the reproducibility in the production of A15 phases with high transition temperatures around 23.degree. K., were also noted by R. E. Somekh, Philosophical Magazine B, Vol. 37, No. 6, pages 713 to 730. In this article, it was stated, that besides the influence of oxygen on the formation of the Nb.sub.3 Ge-phase, there are further parameters still to be properly identified.